Surgical materials and devices

ABSTRACT

PCT No. PCT/FI87/00177 Sec. 371 Date Oct. 14, 1988 Sec. 102(e) Date Oct. 14, 1988 PCT Filed Dec. 29, 1987 PCT Pub. No. WO88/05312 PCT Pub. Date Jul. 28, 1988.Surgical material of resorbable polymer, copolymer, or polymer mixture containing at least partially fibrillated structural units, and use thereof.

Surgical implants with good mechanical strength properties can bemanufactured of resorbable polymeric materials (resorbable composites)which contain resorbable reinforcing elements. Resorbable or absorbablemeans in this connection that the material is metabolized by livingtissues. Such resorbable materials and implants manufactured of them canbe applied e.g. as rods, plates, screws, intramedullary nails etc. forfixation of bone fractures, osteotomies, arthorodesis or joint damages.An advantage of such implants and materials is that they are resorbed(depolymerized to cell nutrients) after the healing of the treatedtissue. Therefore the resorbable implants do not need a removaloperation, like metallic implants in many cases need.

Invention U.S. Pat. No. 4,279,249 describes resorbable implant materialscomprising polyglycolide fibers as reinforcement and polylactide as abinding polymer (as a resorbable matrix). In the patent application FINo. 851 828 are described self-reinforced resorbable materials, wherethe resorbable polymer matrix has been reinforced with resorbablereinforcement elements which have the same chemical element content asthe matrix. Typical reinforcement elements in this connection are fibersor structures which have been constructed of them.

The known resorbable materials reinforced with resorbable organicreinforcement elements have fairly high mechanical strength values.Therefore, such materials can be applied in orthopedics and traumatologyin treatment of cancellous bone fractures, osteotomies, arthrodesis orjoint damages. For example the self-reinforced resorbable materials ofFI No. 851 828 describe bending strengths over 300 MPa (S. Vainionpaa,Thesis, Helsinki 1987), which values clearly are higher than even theaverage strength values of cortical bone. Also the elastic moduli ofknown self-reinforced resorbable composites are quite high, typically ofthe order of magnitude of 10 GPa. So the strength values of thesematerials are clearly better than those of resorbable materials whichhave been manufactured by melt molding techniques.

When one manufactures resorbable polymers, copolymers or polymer alloyrods, profiles, plates, etc. implants by melt molding techniques like byinjection molding or by extrusion, the mechanical properties of theproducts remain on the level which is typical for thermoplasticpolymers. The strength values (like tensile, shear and bending strength)typically do not exceed the value 150 MPa showing, typically, strengthvalues between 40 and 80 MPa and moduli between 1 and 6 GPa. The reasonfor this behavior is the fact that the flow orientation which exists inthe flowing polymer melt is relaxed as a consequence of molecularthermal movements when the melt molded sample is cooled. When it is aquestion of a crystallizable polymer, the sample is crystallized to apartially crystalline, spherulitic structure. So the polymeric materialmanufactured by melt molding typically consists of folded crystallinelamellae (the thickness 100-300 Å, the breath about 1 μm), which aresurrounded by the amorphous polymer. On the other hand, the lamellae canbe thought to consist of mosaic-like folded blocks (the breadth of somehundreds of Å). The lamellae, as a rule, form ribbon-like structureswhich grow from crystallization centers, so-called nuclei, tothree-dimensional spherical spherulitic structures. Because the polymermaterial which has been crystallized with the spherulitic mechanism doesnot show, as a rule, significant orientation of polymer molecules withstrong covalence bonds, its mechanical strength values remain on theabove mentioned level. Only on the surface of the sample molecularorientation can remain because of rapid cooling in the mold (as in thecase of injection molding).

Although the reinforced resorbable composites show considerably betterstrength properties than melt molded resorbable composites, it is oftennecessary to manufacture of resorbable reinforced composites quite bigimplants, like rods, intramedullary nails, screws or plates. This isnecessary because one must secure for the load carrying capacity (likebending or shear load carrying capacity) of the implants a securitymarginal high enough to confirm the stability of the fixation also insuch a case when outer stresses or muscle stresses are directed to thefixated fracture, osteotomy, arthrodesis or joint damage, because theabove mentioned stresses can clearly exceed the weight of the patient.On the other hand, such big implants which the security of the patientdemand, cause quite big operative traumas to the bone tissue and/or tothe soft tissues when the implant e.g. is located into a drill-holewhich has been drilled into the bone or the implant is fastened on thesurface of the bone. With the increasing size of the implant thepossibilities to a foreign body reaction increase, the reaction may growstronger or its duration may be prolonged because the implant and theresorption of the implant cause to the living tissues physical andchemical stresses which are in a direct correlation to the size of theimplant.

So far, the elastic modulus values of resorbable implants which havebeen manufactured of organic materials are at the best the order ofmagnitude of 10 GPa. This is a lower level of elastic modulus than theelastic moduli of cortical bones, which are typically the order ofmagnitude of 20 GPa and can even exceed 30 GPa. When the main goal ofthe surgeon is as good fixation as possible it is advantageous if theelastic modulus of the implant is as near as possible of the elasticmodulus of the bone. In the ideal case the elastic moduli of the bone tobe operated and of the implant are equal. Therefore it is evident thatan efficient fixation of cortical bone, like long bones, needsresorbable organic composite materials which have higher elastic modulusvalues than those of the known materials.

In this invention we have found unexpectedly that by increasing thestrength and elastic modulus values of resorbable polymeric materials byorientation of the molecular structure of the material in such a waythat it is at least partially fibrillated, we get new macroscopicalresorbable self-reinforced implant materials, which have considerablyhigher strength and elastic modulus values than those of the knownresorbable implant materials. When the materials of this invention areapplied as surgical fixation materials, -devices or in manufacturing ofsuch devices one can effectively decrease the operative trauma which theimplant causes and at the same time one can obtain significantly betterfixation than with the known materials. This invention describes atleast partially fibrillated (a) fixation materials for treatment of bonefractures, osteotomies, arthrodesis or joint damages, (b) materials forreconstruction and augmentation of bone tissue and (c) the fixationdevices, reconstruction devices and augmentation devices like rods,plates, screws, intramedullary nails, clamps and chutes manufactured atleast partially of the above materials using as raw material resorbablepolymer, copolymer or polymer mixture. Further this invention describesthe application of at least partially fibrillated resorbable materialsand especially the application of rods, plates, screws, intramedullarynails, clamps or chutes manufactured of the above mentioned materials infixation of bone fractures, osteotomies, arthrodesis or joint damages orin augmentation or reconstruction of bone tissue.

The orientation and fibrillation of spherulitic polymer systems is aprocess, which has been studied extensively in connection with themanufacturing of thermoplastic fibers. E.g. the invention U.S. Pat. No.3,161,709 describes a three phase drawing process, where the melt moldedpolypropylene filament is transformed to a fiber with high mechanicaltensile strength.

The mechanism of the fibrillation is of its main features the followingone (C. L. Choy et al. Polym. Eng. Sci., 23 1983, p. 910). When asemicrystalline polymer is drawn, the molecular chains in thecrystalline lamellae are aligned rapidly along the draw direction. Atthe same time, the spherulites are elongated and finally broken up.Crystalline blocks are torn off from the lamellae and are connected bytaut tie-molecules originating from partial unfolding of chains. Thealternating amorphous and crystalline regions, together with the tauttie-molecules, therefore, form long, thin (ca. 100 Å width) microfibrilswhich are aligned in the draw direction. Since the intrafibrillartie-molecules are created at the interfaces between crystalline blocks,they lie mainly on the outside boundary of microfibrils. Tie-moleculeswhich linked different lamellae in the starting isotropic material arenow connecting different microfibrils, i.e., they become interfibrillartie-molecules locating at the boundary layers between adjacentmicrofibrils. FIG. 1a shows, schematically, how a group of lamellae istransformed to a fibrillar structure (to a fibril which comprises agroup of microfibrils) as a consequence of drawing, and FIG. 1b showsschematically the molecular structure inside of microfibrils and betweenthem. FIG. 1c shows, schematically, the structure of fibrillatedpolymer. This Figure shows several fibrils (one of them has been coloredgrey because of clarity) which comprise several microfibrils with thelength of several micrometers.

The fibrillar structure is already formed at relatively low draw ratiosλ (where λ=the length of the sample after drawing/the length of thesample before drawing). E.g. HD-polyethylene is clearly fibrillated withthe λ value of 8 and polyacetal (POM) with the λ value of 3.

When the drawing of the fibrillated structure is further continued (thisstage of the process is called often ultra-orientation), the fibrillarstructure is deformed by shear displacement of microfibrils, giving riseto an increase in the volume fraction of extended interfibrillartie-molecules. If the drawing is performed at high temperature, theperfectly aligned tie-molecules will be crystallized to form axialcrystalline bridges connecting the crystalline blocks.

The excellent strength and elastic modulus values of the fibrillatedstructure are based on the strong orientation of polymer molecules andmolecular segments into the direction of the drawing (into the directionof the long axis of microfibrils).

Regardless of the high tensile strength of fibrillated fibers theycannot be applied as fixation devices of bone fractures, osteotomies,arthrodesis or joint damages, because thin fibers are flexible and,therefore, they do not show macroscopical bending strength and bendingmodulus and because of their small cross-sectional area they do not havethe necessary shear load carrying capacity which the macroscopicalfixation device must have.

Fibrillation of macroscopical polymeric samples, like rods and tubes, isknown earlier in the case of biostable polyacetal and polyethylene (seee.g. K. Nakagawa and T. Konaka, Polymer 27, 1986, p. 1553 and referencestherein). However, the fibrillation of macroscopical samples ofresorbable polymers has not been known earlier.

At least partial fibrillation of a macroscopical polymer sample can becarried out e.g. by cooling in a capillary tube flowing polymer meltrapidly to the solid state in such a way that the molecular orientationof the flowing molecules cannot relax as a consequence of molecularmotions to a total or partial state of random orientation.

More strong fibrillation and, therefore, also better mechanicalproperties can be achieved by a mechanical deformation (orientation) ofmacroscopical polymer samples. Usually such a mechanical deformation isdone by drawing or by hydrostatic extrusion of material in such aphysical condition (in solid state), where strong molecular structuralchanges of crystalline structure and amorphous structure to fibrillarstate are possible. As a consequence of fibrillation the resorbablepolymeric material which has been manufactured e.g. by the injectionmolding or extrusion and which material initially is mainly spheruliticof its crystalline structure, changes first partially and later ontotally to a fibrillated structure which is strongly oriented in thedirection of drawing or of hydrostatic extrusion. Such a resorbablematerial consists, among other things, of oblong crystallinemicrofibrils and of tie-molecules connecting microfibrils and oforiented amorphous regions. In a partially fibrillated structure theamorphous regions between microfibrils form a more significant part ofthe material than in an ultraoriented material where in the extreme caseamorphous material exists only as crystal defects around the ends of thepolymer molecule chains. When the degree of fibrillation increases in amaterial its strength and elastic modulus values increase many times incomparison to the same values of non-fibrillated material.

Known resorbable composite materials comprise typically randomlyoriented (non-oriented) binding material phase (matrix), which binds toeach other reinforcing elements like fibers which have strongly orientedinternal structure. Such a structure has been shown schematically inFIG. 2, where oriented and non-oriented molecular chains or their partshave been described with thin lines. The strength properties of thebinding phase are significantly weaker than the strength properties ofthe reinforcement elements. Therefore, the strength properties of thecomposite in the direction of orientation of reinforcement elementsincrease when the amount of reinforcement elements in the material isincreased. As a consequence of practical difficulties the amount ofreinforcement elements cannot exceed ca. 70 weight-% of the weight ofthe composite. Therefore, the strength properties of reinforcementelements cannot be utilized totally, because the composite contains alsothe weaker matrix material, which also contributes of its part to thetotal strength of the composite.

By means of orientation and fibrillation it is possible to manufactureof resorbable polymers, copolymers and polymer alloys self-reinforcedcomposites, where nearly the whole mass of material has been oriented ina desired way and where the amount of the amorphous phase is small.Therefore, these materials show very high mechanical strength propertiesin the direction of orientation: tensile strength even 1000-1500 MPa andelastic modulus 20-50 GPa. Accordingly these strength values are clearlybetter than those of known resorbable composites and even about tentimes higher than the strength values of melt molded resorbablematerials.

FIG. 3 shows schematically following structural units which can be seenin the fibrillated structure of polymer fibers and also in the structureof macroscopical, fibrillated polymer samples like rods and tubes:crystalline blocks which are separated from each other by amorphousmaterial (e.g. free polymer chains, chain ends and molecular folds),tie-molecules, which connect crystalline blocks with each other (theamount and thickness of tie-molecules increases with increasing drawratio λ) and possible crystalline bridges between crystalline blocks.Bridges can be formed during drawing when tie-molecules are oriented andgrouped themselves to bridges (C. L. Choy et al. J. Polym. Sci., Polym.Phys. Ed., 19, 1981, p. 335-352).

The oriented fibrillated structure which is shown in FIGS. 1 and 3develops already at so-called natural draw ratios 3-8. When the drawingis continued after this as an ultraorientation at a high temperature,the amount of crystalline bridges can increase very high and in theextreme case bridges and crystalline blocks form a continuouscrystalline structure. The effects of tie-molecules and bridges areoften similar and, therefore, their exact discrimination from each otheris not always possible.

Orientation and fibrillation can be characterized experimentally bymeans of several methods. The orientation function f_(c), which can bemeasured by means of x-ray diffraction measurements, characterizes theorientation of molecular chains of the crystalline phase. f_(c) attainsas a rule already at natural drawing ratios (λ<6) the maximum value 1.The polymeric material with spherulitic structure shows f_(c) <<1.

Birefringence which can be measured by means of polarization microscopeis also a quantity, which describes molecular orientation of molecularchains. As a rule it grows strongly at natural draw ratios (λ<6) andthereafter during ultraorientation more slowly, which shows that themolecular chains of the crystalline phase are oriented into the drawingdirection at natural draw ratios and the orientation of molecules in theamorphous phase continues further at higher draw ratios (C. L. Choy etal. Polym. Eng. Sci., 23, 1983, p. 910-922).

The formation of the fibrillated structure can be shown in many casesillustratively by studying the fibrillated material by means of opticaland/or electron microscopy (see e.g. T. Konaka et al. Polymer, 26, 1985,p. 462). Even single fibrils which consist of microfibrils can be seenclearly in scanning electron microscopy figures which are taken of thefibrillated structure.

Table 1 shows some known resorbable polymers, which can be applied inmanufacturing of resorbable materials and devices of this invention. Apresupposition to an efficient fibrillation is, however, that thepolymer exists in a partially crystalline form. Therefore, suchpolymers, which, because of their physical structure (e.g. configurationstate), are not crystallizable, cannot be effectively fibrillated.

                  TABLE 1                                                         ______________________________________                                        Resorbable polymers                                                           Polymer                                                                       ______________________________________                                        Polyglycolide (PGA)                                                           Copolymers of glycolide:                                                      Glycolide/L-lactide copolymers (PGA/PLLA)                                     Glycolide/trimethylene carbonate copolymers (PGA/TMC)                         Polylactides (PLA)                                                            Stereocopolymers of PLA:                                                      Poly-L-lactide (PLLA)                                                         Poly-DL-lactide (PDLLA)                                                       L-lactide/DL-lactide copolymers                                               Copolymers of PLA:                                                            Lactide/tetramethylglycolide copolymers                                       Lactide/trimethylene carbonate copolymers                                     Lactide/σ-valerolactone copolymers                                      Lactide/ε-caprolactone copolymers                                     Polydepsipeptides                                                             PLA/polyethylene oxide copolymers                                             Unsymmetrically 3,6-substituted poly-1,4-dioxane-2,5-                         diones                                                                        Poly-β-hydroxybutyrate (PHBA)                                            PHBA/γ-hydroxyvalerate copolymers (PHBA/HVA)                            Poly-β-hydroxypropionate (PHPA)                                          Poly-p-dioxanone (PDS)                                                        Poly-σ-valerolactone                                                    Poly-ε-caprolactone                                                   Methylmethacrylate-N-vinyl pyrrolidone copolymers                             Polyesteramides                                                               Polyesters of oxalic acid                                                     Polydihydropyrans                                                             Polyalkyl-2-cyanoacrylates                                                    Polyurethanes (PU)                                                            Polyvinylalcohol (PVA)                                                        Polypeptides                                                                  Poly-β-malic acid (PMLA)                                                 Poly-β-alkanoic acids                                                    ______________________________________                                         Reference:                                                                    P. Tormala, S. Vainionpaa and P. Rokkanen in IVA's Beijer Symposium           "Biomaterials and Biocompatibility", Stockholm, Sweden, August 25-26,         1987.                                                                    

At least partially fibrillated and especially ultraoriented, resorbablepolymer materials are an especially advantageous special case oforiented, self-reinforced resorbable composite materials where theoriented reinforcement elements (crystalline blocks, tie-molecules andcrystalline bridges) form and/or group themselves during the mechanicaldeformation and where the phase which binds the above mentionedstructural units is formed, among other things, of the followingstructural elements: amorphous phase, the interfaces between crystallineblocks and the interfaces between crystalline bridges and micrifibrils,which structural elements are also typically oriented strongly in thedirection of deformation.

The resorbable, at partially fibrillated implant materials andosteosynthesis devices of this invention differ by several unexpectedways from known resorbable implant materials and devices. The materialsand devices of this invention have as a consequence of strongorientation and of at least partially fibrillated structures excellenttensile-, bending- and shear strength properties and elastic modulusproperties. This makes possible to apply in orthopedics and traumatologythinner and smaller rods, plates, screws, nails and clamps etc. thanearlier is known. This decreases advantageously the operative trauma andthe foreign body load caused by the implant to living tissues. Furtherthe excellent mechanical strength- and elastic modulus properties makepossible to apply the materials, implants and devices of this inventionalso in demanding fixation operations of long bone fractures,osteotomies and arthrodesis. It has been found also unexpectedly thatthe implants of this invention retain their mechanical properties inhydrolytic conditions longer than the implants of equal size which havebeen manufactured of the known materials. This makes it also possible toapply the materials and devices of this invention in treatment of suchslowly recovering bone fractures, osteotomies and arthrodesis, where theknown materials and implants cannot be applied.

The at least partially fibrillated rods, tubes, plates etc. profiles ofthis invention can be applied as such as fixation devices e.g. by theways described in the inventions FI Pat. Nos. 69402 and 69403 or thematerials can be formed to different kind of fixation devices likescrews, rods with scaly covering and other profiled structures andclamps or other bended structures, because in this invention has beenalso found unexpectedly that the oriented resorbable materials can behot-worked mechanically at high temperatures without loosening thefibrillated structure. This makes possible e.g. the manufacturing ofespecially strong and tough screws of the at least partially fibrillatedrods of this invention.

It is natural that fibrillated resorbable materials can containadditionally different kinds of additives or auxiliary materials to makethe processing of the material more easy (e.g. stabilizators,antioxidants or plasticizers) or to change its properties (e.g.plasticizers or powder-like ceramic materials) or to make its handlingmore easy (e.g. colours).

The stiff and strong resorbable fixation materials of this invention canbe applied in the form of rods, plates or other profiles also inmanufacturing of bigger fixation devices as reinforcement elements forexample by packing into a cylindrical, oblong injection molding moldfibrillated rods and by filling the mould then by injecting into itsuitable resorbable matrix polymer melt. When the injection is carriedout from one end of the oblong mold, the injected melt flows in thedirection of resorbable reinforcement elements. When the matrix material(polymer melt) flows and solidifies rapidly, into it is formed anadvantageous orientation in the direction of the reinforcement elements.

The stiff and strong fixation rods or plates of this invention can beused also to construct stiff net-like and plate-like structures, whichcan resemble of their mechanical properties more metallic nets than netswhich are manufactured of organic textile fibres. FIG. 4 showsschematically some types of net structures which are constructed ofstiff, strong resorbable rods. Part of the rods has been described aswhite and part as black because of clarity. Such nets can be applied assuch e.g. to treatment of comminuted fractures by combining thecomminuted parts of broken bone to each other and by bending the netaround the parts of broken bone to support it and by fixing the net e.g.with resorbable sutures or clamps. Such nets of this invention can bemanufactured also e.g. by hot-pressing them to curved plates, chutes orbox-like etc. corresponding structures, which can be applied to thereconstruction of bone etc. in such a way that a defect in bone tissue(a hole, a cavity, a cyst, etc.) is filled with tissue compatibleceramic powder like hydroxyapatite or tricalciumphosphate and the curvednet is fixed on the defect to a cover, which immobilizes ceramicparticles and prevents their movements from the defect. Because suchnets of this invention are stiff they function in this connection assignificantly more effective immobilizers than the known flexible netswhich are manufactured of resorbable fibers.

FIG. 5 shows schematically a net structure of this invention which ismanufactured of resorbable rods and which has been bent to the form of achute e.g. by hot pressing. Such a chute can be applied especiallyadvantageously with ceramic materials to augmentation of bone tissue ofalveolar ridges in the following way. First the subperiosteal tunnel ismade surgically below the gingival tissue on the surface of the alveolarridge. The resorbable tube is pushed inside of the tunnel in such a waythat the convex surface of the chute is directed towards the gingivaltissue and the end surfaces of the sides of the chute are placed on thealveolar ridge. This situation has been described schematically in FIG.6 in the case of an operation which is done to the right side of themandible. After installation of the chute it can be filled with ceramicbone graft powder and after that the operation incision can be closed.If necessary, it is possible to place on the same alveolar ridge severalchutes after another. Such a chute prevents the movements of ceramicpowder which has been packed below it. At the same time bone andconnective tissue cells grow from the bone tissue of alveolar ridge andfrom the surrounding soft tissues into the ceramic powder byimmobilizing it at last to a part of the bone tissue of alveolar ridge.The resorbable chute is resorbed at the same time or later.

Ceramic powders and pieces can be applied also in many other ways toaugmentation or reconstruction of bone tissue (as bone graft materials).

Ceramic materials (bioceramics), which are tissue compatible and/orwhich form chemical bonds with bone tissue and/or which promote thegrowth of bone tissue, are e.g. calciumphosphate: apatites likehydroxyapatite, HA, Ca₁₀ (PO₄)₆ (OH)₂ (R. E. Luedemann et al., SecondWorld Congress on Biomaterials (SWCB), Washington, D.C., 1984, p. 224),trade names like Durapatite, Calcitite, Alveograf and Permagraft;fluoroapatites; tricalciumphosphates (TCP) (e.g. trade name Synthograft)and dicalciumphosphates (DCP); magnesiumcalciumphosphates, S-TCMP (A.Ruggeri et al., Europ. Congr. on Biomaterials (ECB), Bologna, Italy,1986, Abstracts, p. 86); mixtures of HA and TCP (E. Gruendel et al.,ECB, Bologna, Italy, 1986, Abstracts, p. 5, p. 32); aluminiumoxideceramics; bioglasses like SiO₂ -CaO-Na₂ O-P₂ O₅, e.g. Bioglass 45S(structure: SiO₂ 45 wt-%, CaO 24,5%, Na₂ O 24,5% and P₂ O₅ 6%) (C. S.Kucheria et al., SWBC, Washington, D.C., 1984, p. 214) and glassceramics with apatites, e.g. MgO 4,6 wt-%, CaO 44,9%, SiO₂ 34,2 %, P₂ O₅16,3% and CaF 0,5% (T. Kokubo et al., SWBC, Washington, D.C., 1984, p.351) and calciumcarbonate (F. Souyris et al., EBC, Bologna, Italy, 1986,Abstracts, p. 41).

The applications of the above ceramic materials as synthetic bone graftshave been studied by different means by using them, for example, both asporous and dense powder materials and as porous and dense macroscopicalsamples as bone grafts. Also ceramic powder - polymer composites havebeen studied in this means (e.g. W. Bonfield et al. SWBC, Washington,D.C., 1984, p. 77).

The resorbable strong and stiff materials of this invention can beapplied in many different ways combined with porous bioceramics tobiocomposites. The mechanical properties, especially the impactstrength, bending strength and shear strength of such composites aresignificantly better than the corresponding properties of porousbioceramics. The invention FI Pat. Appl. No. 863573 describes severalpossibilities to combine resorbable polymeric materials and bioceramics.Those principles can be applied also when the materials of thisinvention are used in combination with bioceramics.

This invention has been illustrated by means of the following examples.

EXAMPLE 1

Poly-L-lactide (PLLA) (M_(w) =600.000) was injection molded tocylindrical rods with a diameter (φ) 4 mm. The rods were drawn to thedrawing ratio λ=7 at temperatures from room temperature to T_(m) -40° C.(where T_(m) =the melting point of the polymer). The fibrillatedstructure of the drawn rods was seen microscopically. Part of the rodswas drawn further to a drawing ratio λ=12 (ultraorientation). Asreference samples were sintered self-reinforced rods (φ=1.5 mm) of PLLAfibers (tensile strength 800 MPa, φ=15 μm) which rods were manufacturedby a method described in FI Pat. Appl. No. 851828.

Following strength values were measured for the injection molded,fibrillated and sintered self-reinforced rods: tensile strength, elasticmodulus and bending strength. The results of measurements are given inTable 1.

                  TABLE 1                                                         ______________________________________                                        Strength properties of PLLA rods                                              Sam-             Rod      Tensile                                                                              Elastic                                                                              Bending                               ple  Manufacturing                                                                             thickness                                                                              strength                                                                             Modulus                                                                              strength                              N:o  method      (mm)     (Mpa)  (GPa)  (MPa)                                 ______________________________________                                        1    Injection   4         80    5.5     70                                        molding                                                                  2    Injection   1.4      560    14     360                                        molding +                                                                     fibrillation                                                                  (λ = 7)                                                           3    Injection   1.2      800    17     470                                        molding +                                                                     fibrillation                                                                  (λ = 12)                                                          4    Self-rein-  1.5      400    10     260                                        forcing                                                                       (sintering)                                                              ______________________________________                                    

Table 1 shows that the strength properties of the fibrillated,resorbable rods of this invention are clearly better than the strengthproperties of the known resorbable materials.

EXAMPLE 2

Resorbable rods of Example 1 (the length 25 mm) were applied to fixationof the arthrodesis of the proximal phalanx of thumb by removing the bothjoint surfaces, by joining the uncovered bone surfaces temporarily toeach other by clamps to an arthrodesis surface, by drilling through thearthrodesis surface two drilling channels and by tapping into thedrilling channels the resorbable fixation rods. 20 patients hadoperations. The average area of the arthrodesis surfaces was ca. 170mm². The calculatory shear load carrying capacity of the fixation was1100N, when two fibrillated rods N:o 2 were applied. The proportion ofdrill channels (which describes the operative trauma) of the arthodesissurface was 1.8%. The corresponding values were for fibrillated rods N:o3 1060N and 1.3% and for sintered rods N:o 4 920N and 2.1%. Accordinglythe fibrillated rods give a stronger fixation than the sintered rods.Also the operative trauma was smaller in the case of the fibrillatedrods. Injection molded rods were not applied in fixation, because theyshould have caused clearly bigger operative trauma (ca. 15%) than theother materials.

EXAMPLE 3

Injection molding was applied to manufacture rods (φ=3.2 mm) of thefollowing resorbable polymers: polyglycolide (PGA) (M_(w) =100.000),glycolide/lactide copolymer (PGA/PLA, the molar ratio 87/13, M_(w)=120.000), poly-β-hydroxybutyrate (PHBA) (M_(w) =500.000) andpoly-p-dioxanone (PDS) (M_(w) =300.000).

Polarization microscopy and scanning electron microscopy showed that,exclusive of a thin surface layer, the rods had a spheruliticcrystalline structure. The melting points (T_(m)) of the materials ofthe rods were measured by differential scanning calorimetry (DSC) andthe following values were obtained for T_(m) : PGA (225° C.), PGA/PLA(180° C.), PHBA (175° C.) and PDS (110° C.). The tensile strengths ofthe rods were: PGA (60 MPa), PGA/PLA (50 MPa), PHBA (30 MPa) and PDS (40MPa). The rods were fibrillated by drawing them at temperatures fromroom temperature to T_(m) -10° C. to drawing ratios λ=8-16. Thediameters of the fibrillated rods were between 0.8 mm and 1.1 mm. Thetensile strengths of the fibrillated rods were: PGA (600 MPa), PGA/PLA(550 MPa), PHBA (400 MPa) and PDS (300 MPa).

EXAMPLE 4

Fibrillated PGA rods of Example 3 and self-reinforced, sintered rods(100 =1.1 mm; which were manufactured of PGA sutures trade name Dexon,size 3-0) which were 50 mm long, were hydrolyzed at 37° C. in distilledwater 5 and 7 weeks. The shear load carrying capacities of fibrillated(f) and sintered (s) rods were after manufacturing f: 570N and s: 300N.After hydrolysis of 5 weeks the corresponding values were f: 160N and s:30N. After 7 weeks hydrolysis the sintered rods had already lost theirshear load carrying capacity, but the fibrillated rods showed still 75Nshear load carrying capacity.

EXAMPLE 5

Fibrillated PGA rods of Example 3 (the length 50 mm, φ1.1 mm) were bentin a mold to clamps shown schematically in FIG. 7a in a bendingtemperature of 180° C. Corresponding self-reinforced clamps weremanufactured of PGA sutures (trade name Dexon, size 3-0) by sinteringthem according to the method of FI Pat. Appl. No. 851828 at elevatedtemperature and pressure in a clamp mold. The tensile load carryingcapacity of fibrillated and sintered clamps was measured by fixing the10 mm long arms of clamps into holes which were in drawing jaws of atensile testing machine and by drawing the clamps according to FIG. 7b.The clamps were broken typically according to FIG. 7b from the base ofthe arm. The fibrillated clamps of this invention showed mean tensileload carrying capacity of 300N and the sintered clamps a correspondingvalue of 120N.

EXAMPLE 6

Fibrillated PLLA rods n:o 4 of Example 1 were compression molded in amold with a screw-like mold cavity at about 160° C. temperature toresorbable 30 mm long screws, with the core thickness of 1.1 mm and theheight of threads 0.5 mm and the distance between the threads 0.8 mm.The tensile load carrying capacity of the screws was 300N. Thecorresponding screws which were manufactured by injection molding ofPLLA showed a tensile load carrying capacity of 80N and thecorresponding self-reinforced, sintered rods which were manufactured ofPLLA fibers of Example 1 showed a tensile load carrying capacity of150N.

EXAMPLE 7

Fibrillated PLLA rods N:o 3 (the length 60 mm, φ1.2 mm) of Example 1were coated with PDLLA (M_(w) =100.000) by immersing the rods in a 5-%acetone solution of PDLLA and by evaporating the solvent. The operationwas repeated so many times that the rods had at last 40 w-% of PDLLA.The coated rods were compressed in a cylindrical mold (the length 60 mmand φ4.5 mm) at 160° C. to cylindrical resorbable rods which showed abending strength of 450 MPa and a bending modulus of 14 MPa.

EXAMPLE 8

Porous hydroxyapatite (HA-) rods (open porosity about 50%, φ=4 mm andthe length 60 mm), which contained on their outer surface 6 longitudinalgrooves shown schematically in FIG. 8a and in a crosssectional FIG. 8b(the cross-section plane A--A of FIG. 8a), and resorbable reinforcingmaterials of this invention were applied to manufacture biocompositerods (intramedullary nails). The used reinforcing element materials werefibrillated PLLA rods (the length 60 mm, φ1.0 mm) of Example 1. AlsoPLLA fibre bundles coated with PDLLA (ca. 0.1 mm thick, slightly twistedbundle of fibers; φ of single fibers 15 μm and tensile strength 800 MPa)were applied as shown below. A 5-% (w/v) acetone solution of PDLLA(M_(w) =100.000) was spread to the grooves of HA-rods and thefibrillated resorbable rods which were immersed in the same solutionwere pushed into the grooves. The rods were adhered into the grooveswhen acetone was evaporated. HA-rods with the fibrillated PLLA rods intheir grooves were coated with PLLA fiber bundle (coated with PDLLA) byfilament winding method. The filament winding was carried out at 150° C.temperature in such a way that the HA-rods were coated with severalfiber bundle layers with different directions so that the fiber bundlelayer was, at the most, 0.4 mm thick. The filament winding was carriedout in such a way that between fiber bundles remained areas of rodsurface without fibers. These uncovered areas of HA-rods could be seenon the surface of biocomposite rods as is shown schematically in FIG.8c. The resorbable reinforced coating of rods was pressed smooth in acylindrical mold (φ=5.0 mm). These biocomposites showed a bendingstrength of 140 MPa, when the bending strength of mere HA-rods was 12MPa.

The above biocomposite rods were applied to fixation of osteotomies ofrabbit femur in the following way. The osteotomy was done with a diamondsaw to the uncovered proximal part of rabbit femur about 1 cm from theneck of the femur. The osteotomy was fixed with clamps. A drill hole(φ=5 mm) was drilled through the greater trocanter vertically into theintramedullary channel of femur. The biocomposite rod was tapped intothe drill hole so that the upper end of the rod was located on the levelof the bone surface. The clamps were removed and soft tissues wereclosed with a resorbable suture. The animals were returned to theircages and after anesthesia they could move immediately freely. 20 testanimals were used. The follow-up time of 6 months showed that all theosteotomies were healed well. Histological examinations ofbone-biocomposite test samples showed growth of bone tissue from femoralbone into the open porosity of HA-rods.

We claim:
 1. Surgical composite comprising a material selected from thegroup of resorbable polymer, resorbable copolymer, and mixtures thereofand further containing oriented, at least partially fibrillatedstructural units (fibrils) which have been induced into the materialproviding said units while said material is in its original nonfibrillarstate by drawing said material in solid state.
 2. The surgical compositeof claims 1 being in the form of a device selected from the groupconsisting of rods, plates, screws, nails, tubes and clamps.
 3. Thesurgical composite of claim 1 which contains at least partiallyultraoriented structural units.
 4. The surgical composite of claim 1having shear strength value of at least 200 MPa and shear modulus valueof at least 4 GPa.
 5. The surgical composite of claim 1 having a bendingstrength value of at least 200 MPa and bending modulus value of at least4 GPa.
 6. The surgical composite of claim 3 having a bending strengthvalue of at least 200 MPa and bending modulus value of at least 4 GPa.7. The surgical composite of claim 3 having shear strength value of atleast 200 MPa and shear modulus value of at least 4 GPa.
 8. The surgicalcomposite of claim 1 the composite comprises a matrix and reinforcementfiber that are the same chemically.